Promoting Silicate Formation by Alkali Chloride Solid Solution during Thermal Conversion of Silicon-Containing Solid Fuels

The development of solid fuel combustion power generation technology in China has been significantly hindered by alkali-related issues including fouling, slagging, and particulate matter emission. Consequently, understanding the formation of silicates in the alkali–silica reaction (ASR) becomes crucial for addressing alkali-related issues associated with solid fuels. However, the complex composition of these fuels and the unknown interaction between multiple alkali chlorides (NaCl and KCl) hinder a comprehensive understanding of ASR. This study investigates the role of alkali chloride solid solution, which contains both Na and K, in silicate formation during ASR. The results reveal that the inclusion of K has three positive effects on silicate formation: the creation of lattice defects, the formation of nano protrusion structures, and the reduction of thermal stability. These effects contribute to higher yields of silicates in ASR, primarily by promoting the formation of SiO4 tetrahedra with three bridge oxygen atoms (Q3 structures). Moreover, a molecular-scale analysis suggests that the presence of K facilitates the dissociation of NaCl within the Q3 and NaCl dimer, thereby favoring silicate formation. This research provides valuable insights into the mechanisms of ASR in the presence of multiple alkali chlorides during the thermal conversion of silicon-containing solid fuels.